The pumping services sector within the oil and gas industry injects fluid into wells to stimulate production or to encase well bore tubulars. The fluids that are pumped usually include various chemicals and solid particulates. The chemicals are added to enhance the properties of the fluids or to make them more compatible with the hydrocarbon bearing formation. The particulates that are added to the fluids are used as propping agents, diverting agents, or as extenders that reduce volumetric cost, change volumetric density, or even enhance properties of the base fluid.
Sands (silicon, ceramic, resin), glass beads, and salts are examples of particulates that are added to fracture fluids, acids, and cements. All of these products come in defined densities and size ranges. The operations that employ these materials are pre-engineered for varying concentrations during the treatment dependent on the desired final results.
Within the industry, it is desirable to monitor the quality of the fluid within the system. This includes monitoring the concentration of particulates within the fluid. Current methods for quality control of the addition of particulates include: batch weighing, both pre- and post-job, mechanical metering during the addition of the particulates, or radioactive density measurements of the fluid slurries during operations.
Batch weighing provides quality control of the cumulative total product used, but does not provide quality control during on the fly operations for pre-engineered programs that vary the rate at which particulates are added during different phases of the injection.
Mechanical metering involves measuring the rate at which the particulate is added and the rate of the fluid prior to addition (clean rate) and then using these rates to calculate the particulate concentration in the slurry. The calculation for concentration is based on the knowledge of the density of the fluid and the particulate material. However, mechanical metering is prone to slippage and inaccuracies due to the efficiencies of the mechanical system being employed. The quality of the measurement is therefore limited.
Another method of measuring concentration is the use of radioactive densitometers, as described for example in the U.S. Pat. No. 5,441,340. The densitometer measures the absolute density of the slurry flowing in the pipe. In these measurements, a flux of photons is typically delivered through a pipe or other containment vessel; the flux of photons transmitted through the vessel and the contained fluid (or other material) is measured at an appropriately located detector, and this can be interpreted to yield fluid density. Given the density of each phase, it is then possible to determine the fractions given the mixture density.
Radioactive density measurements are the most accurate method of concentration measurements. The densities of the fluids and particulates are known prior to pumping and the radioactive density meter reads the absolute density of the slurry from which the particulate concentration can be calculated.
The currently deployed proppant concentration meter uses a radioisotope (“chemical”) source to deliver gamma-rays of appropriate energy in order to provide a density measurement; given the density of the frac fluid and the additive (proppant), it is then simple to determine the proppant volume fraction. However, the deployment of a radioisotope source presents concerns over QHSE (possible exposure to radiation) and particularly security issues (loss or theft of radioactive “chemical” source), as well as licensing and transportation issues.
Various methods and apparatus have been suggested to remove the need for a radioactive source in the densitometer:
One alternative solution taught in the published U.S. Patent application 2004/0007059 is the use of an acoustic sensor.
Another alternative solution is described in the U.S. Pat. No. 6,491,421 and published International Patent application WO 2004/072621, both describing the use of a Coriolis flow meter to determine the density of a fracturing fluid.
In the light of the known state of the art, it is therefore an object of the present invention to provide an apparatus and method to monitor the mixing process of proppants and other solid particles into a fluid or slurry, particularly for use with wellbore service fluids and cementing slurries.